Cyclooxygenase-2 (COX-2), the inducible form of cyclooxygenase, has been found to importantly modify renal development and function. The deletion of the COX-2 gene by homologous recombination results in progressive renal failure, likely due to abnormalities of kidney development and maturation in the postnatal period. In the adult, COX-2 controls renin secretion through its influence on macula densa function. During the last year, our effort has been devoted to addressing the following issues: 1) what factors influence the progression of renal failure in COX-2 knockout mice, and 2) what factors control COX-2 expression in macula densa cells. We developed three congenic lines of COX-2 knockout mice on 129, C57/BL6, and BALB/c backgrounds by backcrossing the original knockout mice with the respective inbred strains for 10 generations and compared the severity of renal failure as well as the development of hypertension among these congenic COX-2 knockout strains. We found that both the genetic background and the gender influence the severity of renal failure and hypertension in COX-2 knockout mice with the most severe phenotype seen in male animals with a 129 background. In addressing the mechanisms of regulation of COX-2 expression in macula densa cells, we examined the interrelationship between COX-2 and nNOS (neuronal form of nitric oxide synthase) in macula densa cells in vitro and in vivo. The critical role of the two enzymes in the regulation of renin secretion was confirmed using COX-2 and nNOS knockout mice. In both strains of mice plasma renin activity was found to be significantly reduced. In a recently established macula densa cell line, nitric oxide stimulated COX-2 expression and in turn the COX-2 product PGE2 inhibited nNOS expression. Tonic inhibition of nNOS expression by a COX product was further supported by the observation that nNOS expression was upregulated in macula densa cells of COX-2 knockout mice. This interaction of COX-2 and nNOS in macula densa cells creates a negative feedback loop that is novel for our understanding of the renin regulating mechanism.